score=int(input('请输入一个整数：'))
if score>100 or score<0:
    print('wrong score.must between 0 and 100.')
elif score>=90:
    print('A')
elif score>=80:
    print('B')
elif score>=70:
    print('C')
elif score>=60:
    print('D')
else:
    print('F')